The emergence of substrates of the FDSOI (“Fully Depleted Silicon On Insulator”) type has given rise to new problems relating to the crystal quality of the silicon substrates from which they are produced.
FIG. 1 schematically illustrates a section of an FDSOI substrate.
Such a substrate successively comprises a handle substrate 1, a buried oxide layer 2 (often referred to by the acronym BOX for “Buried Oxide”) and an ultrathin layer 3 of monocrystalline silicon, which is the active layer, that is to say, in or on which electronic components are intended to be formed.
In the present text, the term “ultrathin” is intended to mean that the thickness of the silicon layer 3 is less than or equal to 20 nm.
The term “FDSOI” refers to an advantageous use of this type of substrate, namely microprocessors. This is because the great thinness of the active layer and, where applicable, the oxide layer, allow the active layer of a transistor formed from this substrate to be fully depleted.
Such a substrate is advantageously, but not exclusively, produced by a method of the SMARTCUT® type, which typically comprises provision of the handle substrate and of a donor substrate from which the active layer will be transferred, the formation of an oxide layer on the surface of the handle substrate and/or of the donor substrate, the formation of a weakened zone in the donor substrate so as to delimit the layer to be transferred, bonding of the substrates via the oxide layer(s), which form the buried oxide, then cleavage of the donor substrate along the weakened zone.
For the formation of FDSOI substrates, donor substrates having an excellent crystal quality are selected.
To this end, donor substrates are selected that are cut from silicon ingots produced according to the NPC (“Near Perfect Crystal”) process, which is the one that generates the fewest defects in the silicon. Donor substrates cut from silicon ingots produced according to the NPC process are used as donor substrates for the production of SOIs for which the active silicon layer has a thickness of 50 nm or more, without introducing an excessive number of defects into this layer.
After cleavage of the donor substrate, however, the active silicon layer 3 may comprise through-defects (holes) D that open into the buried oxide layer 2.
These through-defects are due to defects that were originally present in the silicon ingot from which the donor substrate was formed, and that have a size of between 1 and 20 nm.
Owing to the extreme thinness of the silicon layer, these original defects have a size sufficiently large to pass through the active layer 3, even though they do not affect the silicon layer of a “conventional” SOI substrate, the thickness of which is much greater (of the order of 80 to 100 nm, for example).
These original defects are to be distinguished from oxygen precipitates, from which they differ both morphologically and dimensionally.
For instance, FIG. 2 Panel a presents a photograph of an oxygen precipitate P and FIG. 2 Panel b presents a photograph of an aforementioned original defect D.
In both cases, these SEM (“Scanning Electron Microscopy”) photographs were taken from an FDSOI substrate, the thickness of whose active silicon layer 3 is 12 nm.
Thus, while the oxygen precipitate P has an elongate shape and a length of about 0.2 μm, the original defect D of the silicon has a round shape with a diameter of about 0.08 μm.
Unlike known oxygen precipitates that are generated during heat treatments of a silicon substrate, these original defects are generated during the pulling of the silicon ingot, and they exist therein before any application of a heat treatment.
It should be pointed out that, in the present text, the term “heat treatment” means the introduction of a silicon wafer into a furnace in order to heat it to a high temperature.
During the pulling of the ingot, the silicon is in the liquid state, that is to say, at a temperature above 1,414° C. (melting temperature of silicon), and it cools from 1,414° C. to room temperature when drawn to form an ingot in air, but these steps of producing the silicon ingot are not heat treatments in the sense intended here.
Although the origin of these defects is not precisely known, the current hypothesis is that they are formed during the cooling of the silicon ingot by coalescence of voids, possibly including the creation of an oxygen precipitate inside each void.
The presence of the through-defects D in the active layer 3 (that is to say, a localized absence of silicon) is intolerable because a chip produced on such a defect would be defective.
Even though the heat treatments applied to the silicon when carrying out the SMARTCUT® method probably have the effect of enlarging these defects, these defects would be critical even in the absence of any heat treatment. These defects are difficult to locate because the size of the original defects of the silicon is less than the current detection limit of measurement equipment present on the market, which is on the order of 20 nm. There is, therefore, currently no method for identifying the presence or absence of these defects in a donor substrate, or for measuring the size and the density of these defects, before carrying out the SMARTCUT® method. Thus, there is a need for such method and this is now provided by the present disclosure